Problems with diseased and arthritic hip joints have been with the human race for many years. Since early in the present century, various proposals and operative techniques have been proposed for partial or complete replacement of the hip joint. An extensive article discussing the history of many hip joint prostheses is entitled "THARIES Surface-replacement Arthroplasty for the Arthritic Hip: Rebirth of an Earlier Concept?", by Dr. Harlan C. Amstutz et al., 1977.
When the problem with the hip joint is primarily at the mating surfaces of the joint, replacement of the surfaces is normally to be preferred, whereas, if the head of the femur has been weakened by necrosis or otherwise, the head may be removed, and a prosthesis having a head and a metal stem is implanted into the intramedullary canal at the upper end of the femur. However, the prostheses which have been proposed up to the present time have not been free of problems. Some of the stem-type prostheses have been subject to fatigue fracture, and the acrylic cement which has sometimes been employed to secure prostheses in place, occasionally fails.
With regard to stem-type prostheses, these are normally employed when the femoral head is severely diseased, as noted above, or when a stem in situ has failed and must be replaced. As shown in the article cited above, and in U.S. Pat. Nos. 4,101,985 and 4,141,088, stem-type prostheses normally have collars or shoulders to accurately position the head of the prosthesis relative to the surface of the femur where the head thereof has been removed. However, the presence of the collar or shoulder normally prevents the stem of the prosthesis from seating at full depth firmly into the engagement with the walls of the medullary canal, while fixing the longitudinal position of the prosthesis. However, because the stem does not necessarily engage the walls of the canal, it may shift laterally in its position whether or not it is initially cemented into place. In some cases, the geometry of the stem prosthesis is such that stress concentration points are established, leading to possible fracture of the stem prosthesis.
One medical technique which may usefully be employed in many cases involves removal and reattachment of the trochanter during the operation, with the trochanter being a major protruberance near the upper end of the femur, to which a number of muscles and ligaments are secured. This type of trochanteric osteotomy permits flexibility in the operation, as described in the article by Dr. Amstutz cited above. Now, some proposed femoral prostheses have very large cross-sectional stems toward the upper end of the femur adjacent the trochanter, necessitating the removal of bone in this area. Removal of bone adjacent the trochanter weakens the femur in this area, and makes the trochanter osteotomy procedure less practical; accordingly, femoral prostheses having enlarged upper portions are considered undesirable, from this standpoint as well as for other reasons.
Accordingly, a principal object of the present invention is to provide a stem-type femoral prosthesis which avoids the problems discussed hereinabove.